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    @license twgl.js Copyright (c) 2015, Gregg Tavares All Rights Reserved.
    Available via the MIT license.
    see: http://github.com/greggman/twgl.js for details

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    <title>twgl.js - tunnel</title>
    <style>
      body {
          margin: 0;
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      }
      canvas {
          display: block;
          width: 100vw;
          height: 100vh;
      }
      #b {
        position: absolute;
        top: 10px;
        width: 100%;
        text-align: center;
        z-index: 2;
      }
    </style>
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  <body>
    <canvas id="c"></canvas>
    <div id="b"><a href="http://twgljs.org">twgl.js</a> - tunnel</div>
  </body>
  <script id="tex-vs" type="notjs">
    attribute vec4 a_position;
    attribute vec2 a_texcoord;
    varying vec2 v_texcoord;
    uniform mat4 u_matrix;
    void main() {
      gl_Position = u_matrix * a_position;
      v_texcoord = a_texcoord;
    }
  </script>
  <script id="tex-fs" type="notjs">
    precision mediump float;

    varying vec2 v_texcoord;
    uniform sampler2D u_texture;
    uniform vec4 u_color;
    uniform mat4 u_texMatrix;
    uniform vec2 u_texMod;
    uniform float u_fogNear;
    uniform float u_fogFar;
    uniform vec4 u_fogColor;

    void main() {
      float depth = gl_FragCoord.z / gl_FragCoord.w;
      float fogFactor = smoothstep(u_fogNear, u_fogFar, depth);
      vec2 uv = (u_texMatrix * vec4(v_texcoord, 0, 1)).xy;
      vec4 outColor = u_color * texture2D(u_texture, mod(uv, u_texMod));
      gl_FragColor = mix(outColor, u_fogColor, fogFactor);
    }
  </script>
  <script id="vs" type="notjs">
uniform mat4 u_worldViewProjection;
uniform vec3 u_lightWorldPos;
uniform mat4 u_world;
uniform mat4 u_viewInverse;
uniform mat4 u_worldInverseTranspose;

attribute vec4 a_position;
attribute vec3 a_normal;
attribute vec2 a_texcoord;

varying vec4 v_position;
varying vec2 v_texCoord;
varying vec3 v_normal;
varying vec3 v_surfaceToLight;
varying vec3 v_surfaceToView;

void main() {
  v_texCoord = a_texcoord;
  v_position = (u_worldViewProjection * a_position);
  v_normal = (u_worldInverseTranspose * vec4(a_normal, 0)).xyz;
  v_surfaceToLight = u_lightWorldPos - (u_world * a_position).xyz;
  v_surfaceToView = (u_viewInverse[3] - (u_world * a_position)).xyz;
  gl_Position = v_position;
}
  </script>
  <script id="fs" type="notjs">
precision mediump float;

varying vec4 v_position;
varying vec2 v_texCoord;
varying vec3 v_normal;
varying vec3 v_surfaceToLight;
varying vec3 v_surfaceToView;

uniform vec4 u_lightColor;
uniform vec4 u_ambient;
uniform sampler2D u_diffuse;
uniform vec4 u_specular;
uniform float u_shininess;
uniform float u_specularFactor;
uniform float u_fogNear;
uniform float u_fogFar;
uniform vec4 u_fogColor;

vec4 lit(float l ,float h, float m) {
  return vec4(1.0,
              abs(l),
              (l > 0.0) ? pow(max(0.0, h), m) : 0.0,
              1.0);
}

void main() {
  float depth = gl_FragCoord.z / gl_FragCoord.w;
  float fogFactor = smoothstep(u_fogNear, u_fogFar, depth);
  vec4 diffuseColor = texture2D(u_diffuse, v_texCoord);
  vec3 a_normal = normalize(v_normal);
  vec3 surfaceToLight = normalize(v_surfaceToLight);
  vec3 surfaceToView = normalize(v_surfaceToView);
  vec3 halfVector = normalize(surfaceToLight + surfaceToView);
  vec4 litR = lit(dot(a_normal, surfaceToLight),
                    dot(a_normal, halfVector), u_shininess);
  vec4 outColor = vec4((
  u_lightColor * (diffuseColor * litR.y + diffuseColor * u_ambient +
                u_specular * litR.z * u_specularFactor)).rgb,
      diffuseColor.a);
  gl_FragColor = mix(outColor, u_fogColor, fogFactor);
}
  </script>
  <script src="../3rdparty/chroma.min.js"></script>
  <script type="module">
    /*eslint no-unused-vars:0*/
    import * as twgl from '../dist/4.x/twgl-full.module.js';

    twgl.setDefaults({attribPrefix: "a_"});
    const m4 = twgl.m4;
    const gl = document.querySelector("#c").getContext("webgl");
    const programInfo = twgl.createProgramInfo(gl, ["vs", "fs"]);
    const texProgramInfo = twgl.createProgramInfo(gl, ["tex-vs", "tex-fs"]);

    function createUnitQuadsBufferInfo(gl, across, down) {
      const numQuads    = across * down;
      const numVertices = 4 * numQuads;
      const numIndices  = 6 * numQuads;

      const positions = twgl.primitives.createAugmentedTypedArray(3, numVertices);
      const ids       = twgl.primitives.createAugmentedTypedArray(2, numVertices, Uint16Array);
      const indices   = twgl.primitives.createAugmentedTypedArray(3, numIndices, Uint16Array);

      const verts = [
        -1, -1, -1,
         1, -1,  0,
        -1,  1,  0,
         1,  1,  1,
      ];

      let ndx = 0;
      for (let yy = 0; yy < down; ++yy) {
        for (let xx = 0; xx < across; ++xx) {
          positions.push(verts);
          ids.push(xx, yy, xx, yy, xx, yy, xx, yy);
          indices.push(ndx + 0, ndx + 1, ndx + 2, ndx + 2, ndx + 1, ndx + 3);
          ndx += 4;
        }
      }

      return twgl.createBufferInfoFromArrays(gl, {
        position: positions,
        id: ids,
        indices: indices,
      });
    }


    const segments = 20;
    const cyAcross = 12;
    const cyDown = 25;
    const bufferInfo = twgl.primitives.createCubeBufferInfo(gl, 2);
    const cylinderBufferInfo = twgl.primitives.createCylinderBufferInfo(gl, 1, 200, cyAcross, cyDown, false, false);

    const fbSize = 1024;
    const framebufferInfo = twgl.createFramebufferInfo(gl, undefined, fbSize, fbSize);

    const tex = twgl.createTexture(gl, {
      min: gl.NEAREST,
      mag: gl.NEAREST,
      src: [
        255, 255, 255, 255,
        192, 192, 192, 255,
        192, 192, 192, 255,
        255, 255, 255, 255,
      ],
    });

    const uniforms = {
      u_lightWorldPos: [1, 8, -10],
      u_lightColor: [0.2, 0.5, 1.0, 1],
      u_ambient: [0, 0, 0, 1],
      u_specular: [1, 1, 1, 1],
      u_shininess: 50,
      u_specularFactor: 1,
      u_diffuse: tex,
      u_fogColor: [1, 1, 1, 1],
      u_fogNear: 5,
      u_fogFar: 25,
      u_world: m4.identity(),
    };

    const cylinderUniforms = {
      u_texture: framebufferInfo.attachments[0],
      u_matrix: m4.identity(),
      u_color: [1, 1, 1, 1],
      u_texMod: [1, 1],
      u_texMatrix: m4.identity(),
      u_fogColor: [1, 1, 1, 1],
      u_fogNear: 0.1,
      u_fogFar: 190,
    };

    function lerp(a, b, t) {
      return a + (b - a) * t;
    }

    let useFramebuffer = true;

    function switchMode() {
      useFramebuffer = !useFramebuffer;
    }

    window.addEventListener('keypress', switchMode, false);
    window.addEventListener('click', switchMode, false);
    window.addEventListener('touchstart', switchMode, false);

    let currentBaseColor;
    let targetBaseColor;
    let baseColorScale;
    const colorFadeDuration = 5;
    const colorChangeDuration = 10;
    let colorFadeTimer;
    let colorChangeTimer;

    function pickNewColor() {
      currentBaseColor = targetBaseColor || chroma.hsv(Math.random() * 360, 1, 1);
      targetBaseColor = chroma.hsv(Math.random() * 360, 1, 1);
      baseColorScale = chroma.scale([currentBaseColor, targetBaseColor]).mode('lab');
      colorFadeTimer = colorFadeDuration;
      colorChangeTimer = colorChangeDuration;
    }
    pickNewColor();

    let then = 0;
    function render(time) {
      time *= 0.001;
      const deltaTime = time - then;
      then = time;

      twgl.resizeCanvasToDisplaySize(gl.canvas);

      let aspect;
      if (useFramebuffer) {
        twgl.bindFramebufferInfo(gl, framebufferInfo);
        aspect = 1;
      } else {
        twgl.bindFramebufferInfo(gl, null);
        aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
      }

      colorFadeTimer -= deltaTime;
      colorChangeTimer -= deltaTime;
      if (colorChangeTimer <= 0) {
        pickNewColor();
      }

      const colorLerp = 1 - Math.max(0, colorFadeTimer / colorFadeDuration);
      const fadeColor = baseColorScale(colorLerp);
      uniforms.u_fogColor = fadeColor.gl();
      const fc = uniforms.u_fogColor;
      cylinderUniforms.u_fogColor = fc;

      gl.cullFace(gl.BACK);
      gl.enable(gl.CULL_FACE);
      gl.enable(gl.DEPTH_TEST);
      gl.clearColor(fc[0], fc[1], fc[2], fc[3]);
      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

      const projection = m4.perspective(30 * Math.PI / 180, aspect, 0.5, 30);
      const eye = [1, 0, -16];
      const target = [0, 0, 0];
      const up = [0, 1, 0];

      const camera = m4.lookAt(eye, target, up);
      const view = m4.inverse(camera);
      const viewProjection = m4.multiply(projection, view);
      uniforms.u_viewInverse = camera;

      gl.useProgram(programInfo.program);
      twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
      for (let ii = 0; ii < 25; ++ii) {
        const x = ii % 5;
        const y = ii / 5 | 0;
        const world = uniforms.u_world;
        m4.identity(world);
        m4.translate(world, [0, 0, 0], world);
        m4.rotateY(world, time * 0.43, world);
        m4.rotateZ(world, time * 0.37, world);
        m4.translate(world, [x * 3 - 7 - 3, y * 3 - 5 - 3, 0], world);
        m4.rotateX(world, time * 0.21 + ii, world);
        m4.rotateY(world, time * 0.53 + ii, world);

        uniforms.u_world = world;
        uniforms.u_worldInverseTranspose = m4.transpose(m4.inverse(world));
        uniforms.u_worldViewProjection = m4.multiply(viewProjection, world);
//        uniforms.u_lightColor[0] = x / 4;
//        uniforms.u_lightColor[1] = y / 4;
//        uniforms.u_lightColor[2] = 1 - (x + y) / 8;

//        uniforms.u_lightColor[0] = Math.max(x, y) / 5;
//        uniforms.u_lightColor[1] = Math.max(y, x) / 5;
//        uniforms.u_lightColor[2] = 1;// - (x + y) / 8;
        uniforms.u_lightColor = [2, 2, 2, 1];

        twgl.setUniforms(programInfo, uniforms);
        twgl.drawBufferInfo(gl, bufferInfo);
      }

      if (useFramebuffer) {
        twgl.bindFramebufferInfo(gl, null);

        gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
        gl.cullFace(gl.FRONT);
        gl.useProgram(texProgramInfo.program);

        aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
        const mat = cylinderUniforms.u_matrix;
        m4.perspective(30 * Math.PI / 180, aspect, 0.5, 190, mat);
        m4.translate(mat, [0, 0, -100], mat);
        m4.rotateZ(mat, -time * 0.1, mat);
        m4.rotateX(mat, Math.PI * 0.5, mat);

        const cmat = cylinderUniforms.u_texMatrix;
        m4.identity(cmat);
        m4.translate(cmat, [time * 1, time * 2, 0], cmat);
        m4.scale(cmat, [lerp(6, cyAcross, Math.sin(time) * 0.5 + 0.5), cyDown, 1], cmat);
        m4.rotateZ(cmat, time * 0.1, cmat);
        cylinderUniforms.u_texMod[0] = 0.5;
        cylinderUniforms.u_texMod[1] = 0.5;

        twgl.setBuffersAndAttributes(gl, texProgramInfo, cylinderBufferInfo);
        twgl.setUniforms(texProgramInfo, cylinderUniforms);
        twgl.drawBufferInfo(gl, cylinderBufferInfo);
      }

      requestAnimationFrame(render);
    }
    requestAnimationFrame(render);

  </script>
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